pub mod mod_a {
    #[derive(Debug)]
    pub struct A {
        pub number: i32,
        name: String,
    }
    impl A {
        pub fn new_a() -> A {
            A {
                number: 1,
                name: String::from("A老大"),
            }
        }
        pub fn print_a(&self) {
            println!("number = {}, name = {}", self.number, self.name);
        }
    }
    pub mod mod_b {
        pub fn print_b() {
            println!("mod B");
        }
        pub mod mod_c {
            pub fn print_c() {
                println!("mod C");
                // 如果想要使用模块B中的print_b，需要使用super关键字
                super::print_b();
            }
        }
    }
}